Modeling of several aspects of muscle contraction is presently underway. First, we are quantitatively modeling our new cross-bridge model which is based on the concept that the myosin cross-bridge oscillates between states weakly bound to actin with a preferred angle of about 90 degrees and states strongly bound to actin with a preferred angle of about 45 degrees. The former states have bound APT or ADP + Pi while the latter states have no bound Pi. The concept of two major types of cross-bridge states is combined with the biochemical concept of rapid equilibria between attached and detached states and the physiological concept of an elastic cross-bridge to allow a model in which the myosin does not detach from actin during each cycle of cross-bridge action but does mechanically detach when the muscle is shortening. In addition to modeling cross-bridge action in vivo, we are currently modeling the cooperative actions of the troponin-tropmyosin actin complex, in particular the cooperative binding of myosin ADP and the cooperative effect of troponin-tropomyosin on the steady-state actin-activated myosin ATPase activity. Finally, we are comparing our current models of muscle contracton with other energy transduction systems such as ion transport to determine similarities and differences between the various systems.